1. Field of the Invention
The present invention relates to an image forming apparatus, and more specifically relates to a high-voltage power supply apparatus employed in an image forming apparatus.
2. Description of the Related Art
In an electrophotographic image forming apparatus, transfer of a toner image is expedited by applying a direct current bias voltage to a transfer roller formed by wrapping a roller-like conductive rubber around a metal shaft. In order for transfer to be performed well, ordinarily, electric current of high voltage (voltage of at least several hundred volts greater than the voltage of a commercial power source) and about 10 μA is caused to flow to the transfer roller.
In order to generate this sort of high voltage, conventionally, a wire wound type electromagnetic transformer is used. However, an electromagnetic transformer is an obstacle to reducing the size and weight of a high-voltage power supply apparatus. Consequently, use of a piezoelectric transformer (a piezoelectric ceramic transformer) is being investigated. With a piezoelectric transformer, high voltage can be generated with greater efficiency than an electromagnetic transformer, and moreover, a mold process for isolating electrodes of a primary side and a secondary side is also unnecessary. Therefore, a piezoelectric transformer has the advantage of allowing reduction of the size and weight of high-voltage power supply apparatuses.
In the circuit design of an ordinary piezoelectric transformer type high-voltage power supply apparatus, the voltage that is output is controlled according to frequency (Japanese Patent Application Laid-open No. H11-206113).
However, in a conventional circuit design, spurious frequencies are generated in the range of resonance frequencies. When a spurious frequency is generated, the output voltage becomes unstable in response to variation of load or minute changes in transformer performance, and thus it becomes difficult to obtain a high quality image. Therefore, it is desirable to decrease the output voltage at a spurious frequency.
The inventors of the present application investigated inserting a series resistor in a current path that runs from a rectifier circuit provided in a latter stage of a piezoelectric transformer. However, the inventors learned that when a series resistor is inserted, there is the drawback that not only the voltage at a spurious frequency, but also the highest voltage at a resonance frequency f0 decreases. Furthermore, the inventors also investigated a circuit design in which the reduction in the highest voltage at the resonance frequency f0 is suppressed by switching the series resistor during high-voltage output with a relay. However, this design as well could require the addition of expensive and/or complicated circuits.